The following is a tabulation of some prior arts that presently appear relevant:
U.S. PatentsPatent NumberKind CodePublication DatePatentee3,049,435A1962 Aug. 14Shwayder7,303,030B22007 Dec. 4Lockwood et al.8,056,652B22011 Nov. 15Lockwood et al.5,876,793A1999 Mar. 2Sherman et al.U.S. Patent Application PublicationsPublication NumberKind CodePublication DateApplicant20060081681A12006 Apr. 20Pipkin
Hardfacings (they are also referred to as hardbandings for tubulars of earth boring tools such as drill pipes) are hard coatings over metallic substrate surfaces and have higher hardness than metallic substrates, so as to improve their abrasive wear and erosion resistances. Hardfacings are widely used in machinery, mining, agriculture, construction, and oil and gas industries. Hardfacings can be divided into thin and thick ones. Thin hardfacings usually have a thickness of not more than 50 μm. Typical techniques for preparing the thin hardfacings include physical vapor deposition (PVD), chemical vapor deposition (CVD), electroless/electrical plating, chemical heat treatments such as carburizing, carbonitriding, nitriding, and boriding, surface mechanical processing, etc. On the other hand, the thick hardfacings generally have a thickness of more than 50 μm, up to several millimeters, or even larger. The thick hardfacings are usually prepared by welding or brazing methods. The main techniques include laser cladding, plasma transferred arc (PTA) welding, consumable and non-consumable electric arc welding, oxyacetylene flame welding and brazing, or furnace brazing, etc. The thick hardfacings are made of either uniform metals and alloys, or metallic matrix composites (MMCs) with discrete hard phase particles as reinforcements. Tungsten carbide particles are the most commonly used reinforcements in the MMC hardfacings, due to their extraordinarily high hardness.
MMC hardfacings generally have 40-80 wt. % of tungsten carbide particles embedded in balanced binder alloys. The hard tungsten carbide particles act as anti-wear components and the tough binder matrices are their holders. The binder alloys are generally ferrous metals (iron, cobalt, and nickel) or their alloys. There are five common methods to prepare such the hardfacings. (i) Dropping particles: tungsten carbide particles alone may be dropped into a molten pool during welding such as gas metal arc welding (GMAW) or gas tungsten arc welding (GTAW) operations. (ii) Spraying powder: premixed powder materials of tungsten carbide particles and binder alloy particles may be sprayed onto a substrate surface during welding or brazing. This process is used in oxyacetylene spray & fuse, laser cladding, or PTA. (iii) Preformed coating: a slurry, paste, or cloth containing carbide particles and binder materials may be applied to a substrate surface and subsequently, it is heated for brazing in a furnace or with induction coils. Usually, such the brazing processes are performed in vacuum or an inert atmosphere. (iv) Rod: a welding or brazing rod may be prepared by placing a mixture of tungsten carbide particles, deoxidizer, alloying elements, and binder material into a metal tube. A substrate may be hardfaced by progressively melting a welding or brazing rod onto a selected surface of the substrate and allowing the melted material to solidify and form a desired hardfacing. (v) Rope: a brazing rope consists of a metallic wire as a core and its skin material comprising a mixture of tungsten carbide particles, deoxidizer, alloying elements, and binder materials. A substrate may be hardfaced by progressively melting a brazing rope onto a selected surface of the substrate and allowing the melted material to solidify and form hardfacing. An oxyacetylene torch is usually used for heating.
The tungsten carbide herein includes cast tungsten carbide (also known as fuse tungsten carbide), carburized tungsten carbide, macro-crystalline tungsten carbide, and sintered tungsten carbide. The cast tungsten carbide is eutectic WC and W2C. The carburized tungsten carbide is a product of a solid-state diffusion of carbon into tungsten metal powders or particles at high temperatures in a protective atmosphere. Usually, the carburized tungsten carbide is polycrystalline monotungsten carbide (WC). The macro-crystalline tungsten carbide is essentially stoichiometric tungsten carbide (WC) created by a thermite process. Most of the macro-crystalline tungsten carbide is in the form of single crystals. The sintered tungsten carbide (also known as cemented tungsten carbide) is a composite of tungsten carbide and a metal binder made through powder metallurgy method. The metallic binder is commonly cobalt, and sometimes nickel, iron, or their alloys. Recently, binderless sintered tungsten carbide containing hardly any metallic binder was developed. Various tungsten carbides have different mechanical, physical, and chemical properties, such as hardness and toughness. All these types of tungsten carbides are commonly used in hardfacings in either single or combined forms, which depends on different application occasions.
A portion of tungsten carbide material may be lost by vaporization, oxidation, or dissolution during welding or brazing processes, when manufacturing MMC hardfacings with tungsten carbide particles as reinforcements. Ferrous binder materials (iron, cobalt, nickel, or their alloys) act as a dissolving agent for the tungsten carbide. Since the price of the tungsten carbide is high, the loss of this material is expensive. More seriously, dissolved tungsten and carbon atoms would enter into a binder matrix, and as a result, would lead to formation of some detrimental brittle phases such as eta phase (M6C) precipitates, which would reduce the toughness of the binder and embrittle the matrix substantially.
U.S. Pat. No. 3,049,435 discloses a process for applying tungsten carbide particles onto a workpiece surface. The tungsten carbide particles are coated with a nickel or nickel-phosphorus alloy. This coating acts as a barrier coating for tungsten carbide particles to protect them from degeneration. Thus, there is a reduced loss of the tungsten carbide in application of the particles to a workpiece. However, it is noted that the nickel or nickel-phosphorus alloy as a metallic coating is vulnerable in a binder alloy melt and it would disappear very soon during welding or brazing.
U.S. Pat. No. 7,303,030, B2 discloses a technique of coating tungsten carbide particles with a barrier layer for improving hardfacing material. The hardfacing with the tungsten carbide particles with a barrier coating as reinforcements is for drilling bits. The tungsten carbide used is sintered tungsten carbide. The carbide size is 420 to 1190 μm (16 to 40 mesh). The barrier coating is a metal layer that is formed from at least one of cobalt, nickel, iron, or alloys thereof, whose thickness is from 5 to 76 μm. The methods for coating tungsten carbide particles are electroless plating, electrical plating, and atmospheric pressure CVD. The invention prevents or reduces the formation of a “halo” around a carbide that indicates the dissolution of the carbide.
Metals or alloys as coating materials may not have sufficiently high melting points and especially, their solubility to binder materials (iron, cobalt, nickel, or their alloys) is high. Thus, it can be deduced that as a barrier, the metal or alloy coatings would have a limited protection for the tungsten carbide particles.
U.S. Pat. No. 8,056,652 B2 discloses a technique of coating tungsten carbide particles with a barrier layer using atomic layer deposition for improving hardfacing material onto drilling bits. The tungsten carbide particles comprise cemented tungsten carbide particles. The barrier coating comprises a layer of ceramic material disposed on the abrasive particles. The barrier coating comprises a thickness ranging from about 1 to 500 nanometers (nano-sized coating). The ceramic layer includes metal oxides, carbides, nitride, etc. The invention reduces coating thickness to a nanometer scale.
In U.S. Pat. No. 8,056,652 B2, the nano-sized oxide coating may survive attack from a binder alloy melt during hardfacing, as the oxides have high thermodynamic stability and very little chemical interaction with a binder matrix. But, the oxides have an incoherent interface with a binder of iron, cobalt, nickel, or their alloys, and thus, their bonding strength is low. On the other hand, the nano-sized carbides and borides are too thin to survive attack from a binder alloy melt during hardfacing, even though they can generate a coherent or semi-coherent interface with a binder alloy.
In U.S. Pat. No. 8,056,652 B2, the coating of the carbide particles may have multiple layers, i.e., an interior ceramic layer and an exterior metallic layer. However, as discussed before, a metallic coating layer is vulnerable to a binder alloy melt and it would disappear very soon during welding or brazing due to the attack from a binder alloy melt. No much protection from an exterior metallic coating layer is expected.
While these coatings may improve some properties, these hardfacing compositions may have limitations. Accordingly, there exists a continuing need for improving hardfacing materials, especially for laser cladding, PTA, and electric arc welding where extremely high heat inputs are involved during welding or brazing processes.
Embodiments of this disclosure will improve hardfacing qualities and performances including abrasion and erosion wear resistances, and especially impact resistance with higher toughness. The improved hardfacing material will find wide applications in machinery, mining, agriculture, construction, and oil and gas industries.